1. Field of the Invention
The present invention relates to integrated circuits obtained through utilizing CMOS technologies, i.e. utilizing simultaneously upon a single semiconductive substrate N channel field effect transistors and P channel field effect transistors.
Numerous logic circuits known according to the prior art require the use of static bistable flip-flop circuits (latched D flip-flops).
FIG. 1 represents a conventional static bistable flip-flop circuit, obtained by utilizing CMOS technology.
This flip-flop circuit comprises an input E, an output S, and between the two, an assembly in series comprising a first switch, a first inverter stage, a second switch and a second inverter stage. The flip-flop circuit is controlled by two complementary periodic clock signals H and H.
The first switch comprises a MOS N channel transistor T1 parallel with a MOS P channel transistor T2, both these transistors being positioned between the input terminal E and the input A of an inverter I1 of the first inverter stage. The gate of the N channel transistor T1 is controlled by the clock signal H and the gate of the P channel transistor T2 is controlled by the complementary clock signal H.
The second switch comprises a MOS N channel transistor T3 parallel with a MOS P channel transistor T4; both these transistors are positioned between the output B of the inverter I1 and the input C of an inverter I2 of the second inverter stage. The gate of the N channel transistor T3 is controlled by the complementary clock signal H; the gate of the P channel transistor T4 is controlled by the clock signal H.
The first inverter stage comprises, between the circuit connecting points A and B, in addition to the inverter I1 which has its input connected to the point A and its output to the point B, a holding loop comprising a third inverter I3 and a third switch. The third inverter has its input connected to the point B and its output connected to the point A through the third switch. The third switch comprises a MOS N channel transistor T5 parallel to a MOS P channel transistor T6. The gate of the N channel transistor T4 is controlled by the complementary clock signal H; the gate of the P channel transistor T6 is controlled by the clock signal H.
In the same way, the second inverter stage comprises, between the connecting point C and the output S of the flip-flop, on the one hand the inverter I2 which has its input connected to the point C and its output connected to the output S, and on the other hand, a holding loop comprising a fourth inverter I4 and a fourth switch. The fourth inverter has its input connected to the terminal S and its output connected to the fourth switch; the fourth switch is furthermore connected to the point C and it comprises in parallel a N channel transistor T7 and a P channel transistor T8; the gate of the N channel transistor T7 is controlled by the clock signal H. The gate of the P channel transistor T8 is controlled by the complementary clock signal H.
The inverters are each generally constituted by two complementary transistors in series between two power supply terminals (not represented) supplying a voltage VCC. The input of the inverter is taken upon the connected gates of the two transistors, the output upon their drains.
The operating mode of the flip-flop is the following:
(a) first half-period: H at the logic level 1 PA0 (b) second half-period; H at the logic level 1; PA0 (c) first half-period of the following period;
T1 and T2 are conductive, T3 and T4 blocked, T5 and T6 blocked. The logic level of the input E is inverted at the output B of the inverter I1. The operation of the second inverter stage is in no way influenced by the input E since the transistors T3 and T4 are blocked.
T1 and T2 are blocked; the input E in no way impairs the state of the flip-flop; T5 and T6 are rendered conductive and the preceding state of the connecting point B is confirmed by the application of the complement of this state upon the connecting point A through the inverter I3 and the transistors T5, T6. This state of the connecting point B, confirmed by this locking loop, is furthermore transmitted to the input C of the second inverter I2 and the output S thus assumes the complementary state of that of the connecting point B.
H is the logic level 1. The state of the output S is confirmed by the locking loop of the second stage since T7 and T8 become conductive at the same time as T3 and T4 are blocked.
This conventional flip-flop assembly presents a certain number of drawbacks that the present invention to a large extent eliminates.
(1) The mode of operating described herein-above is only effective if the signals H and H are strictly complementary, since, if there is partial overlapping of the signals H and H, there is a risk of direct transmission of the input signals occurring towards the output without locking by one of the groups of transistors T1, T2 or T3, T4: for example if H and H present an overlapping lapse of time during which H and H are practically both at the level 1, T1 and T3 are simultaneously conductive, which is not admissible; this also happens if the signals H and H have insufficiently steep rising fronts.
When the operating frequency increases, it becomes extremely difficult to form complementary signals H and H without overlapping (i.e. perfectly complementary signals presenting very steep rising and falling fronts).
(2) In any case, it is very difficult to obtain signals having very steep rising fronts when these signals are applied to a capacitive load; the gates of the various transistors controlled by the clocks H and H are effectively capacitive loads; the greater the number of transistors, the more difficult it becomes to preserve the steep fronts for the clock signals. It should not be overlooked that in practice an integrated circuit could comprise numerous flip-flops controlled by the same clock signals. It is therefore worth-while to reduce the number of transistors utilized.
(3) One problem which must be taken into account in designing a flip-flop, or more generally in designing an inverter (and the flip-flop represented in FIG. 1 comprises four inverters), is the input voltage level from which the inverter flips (in one direction or the other): it is necessary that the inverter flips completely for an intermediary input voltage value between the voltage that defines the low logic level and that which defines the high logic level.
For example, for a logic circuit theoretically operating between 0 and 5 volts, it is possible to have an arrangement whereby the flipping over occurs at 2.5 volts. Since the mobility of the electrical charges in the P channel transistors is lower than the in the N channel transistors, this problem is generally overcome by providing P channel transistors twice as great as the N channel transistors in the inverters. But this arrangement increases the overall bulk of the flip-flop circuit and constitutes a major drawback.
In order to overcome these drawbacks, the present invention proposes to dispense with utilizing for certain switches two complementary transistors in parallel. In order to fully understand the unexpected aspect of this proposition, it should be recalled that in CMOS technology, efforts are made wherever possible to utilize as switch an assembly of two complementary MOS transistors actuated by two complementary clock signals; this is true, in particular, each time that a switch has to be placed upstream of an inverter.
The reason for this quasi-universal arrangement is the following: if the switch had only been constituted by a single N channel transistor there would be no problem in transmitting a logic level (0 volt) through the switch towards the input of the inverter; if there is 0 volt at the input of the N channel transistor while this transistor is rendered conductive, there is also 0 volt upon its output. But, in order to transmit a high level (5 volts) this is no longer the case; in fact, if there is a value of 5 volts at the input of the N channel transistor while it is rendered conductive, there will only be 5 volts-Vt at its output, Vt being the threshold voltage of the transistor. The inverter, according to CMOS technology, is constituted by a P channel transistor in series with a N channel transistor, the gates of the two transistors being connected together in order to constitute the input of the inverter; applying 5 volts-Vt at the input of the inverter is therefore certainly sufficient to render the N channel transistor conductive, but it is insufficient to suitably block the P channel transistor. The inverter does not operate correctly; it consumes power. This is the reason why provision is always made upstream from a CMOS inverter, since it has been established that this problem exists, for switches provided with two complementary transistors in parallel, the two transistors being actuated by complementary clock signals. When the second transistor of the switch must transmit a high level (5 volts) it transmits correctly this high level without causing it to drop by a threshold voltage.
According to the invention, a static bistable flip-flop circuit, produced by CMOS technology, is foreseen, comprising an input, an output, a first inverter, a first switch connecting the input of the flip-flop to the input of the first inverter, a second inverter, a second switch connecting the output of the first inverter to the input of the second inverter, a third inverter, a third switch connecting the output of the third inverter to the input of the first inverter, a fourth inverter, a fourth switch connecting the output of the fourth inverter to the input of the second inverter, the first switch and the fourth switch being simultaneously controlled by a first clock signal and the second and the third switches being simultaneously controlled by a second clock signal in phase opposition with the first, characterized in that:
each switch is constituted by a single field effect N channel transistor, the gate of which is controlled by the first clock signal (first and fourth switches) and by the second clock signal (second and third switches);
two MOS P channel transistors are foreseen, the first P channel transistor being placed in parallel on the third switch and having its gate connected to a first circuit connecting point so that it is conductive at least when the output of the first inverter is at a low logic level, and the second P channel transistor being placed in parallel on the fourth switch and having its gate connected to a second circuit connecting point so that it is conductive at least when the output of the second inverter is at a low logic level.
The first circuit connecting point can be the output of the first inverter or a point having a reference voltage constituting a low logic level, for example earth ground.
The second circuit connecting point can be the output of the second inverter or a point having a reference voltage constituting a low logic level, for example earth ground.